Despite what everyone says about Apple and Onion, I, an enthusiast in animation, with a particular fondness for cartoons, find Apple and Onion to be not as bad as it has been portrayed by most reviews. Despite some shows bringing a bad rep to the buddy-buddy adventures together, like Disney's short-lived Pickle and Peanut (pretty forgettable) or Nickelodeon's Sanjay and Craig, Apple and Onion does NOT fall into that category of bad shows. Its art style and animation is pretty good. The style is distinct and the art is subjectively captivating, expressive, and simplistic (not in a bad way).
I think many are complaining about the humor or the demographic, but to the average viewer, I would honestly say that if you watch it with an open mind, I don't think you'll consider the show childish or on the basis of toddler humor. I think there is definitely childish jokes in there, however, it isn't the foundation of the show's humor. I think the average viewer could appreciate the humor (humor is subjective though).
Each character also has a unique personality and design There is an obvious contrast between Apple and Onion, Apple being more dependent, reckless, and a follower (for example, in "Onionless," Apple's desperation and helplessness is obvious without Onion there to support his needs), while Onion is a more nuturing, organized, and dependable (This can be noted in "Apple's in Charge," where Onion is always in charge of Patty's Dollar Store and always earns the rewarding stickers for maintaining the store well). And yet, they are still friends because they support and love each other.
Overall, Apple and Onion is a great show and doesn't deserve as much disdain as it has received. It's executed well, with quality art, animation, and characterization. The humor is nicely done (not nearly as bad as everyone makes it out to be), and you should give it a shot with an open mind before forming an opinion. The show is a 'cutsier,' nicer, vibe than Teen Titans Go, which is airing pretty much all the time.
tl;dr: it's good